Green’s Dictionary of Slang

Neptune’s daughter n.

(US) water.

1935		A.J. Pollock Und. Speaks.
1949		Monteleone Criminal Sl. (rev. edn).
1960		J. Franklyn Dict. of Rhy. Sl.
1976		(ref. to 1930s–70s) R. Barnes Coronation Cups and Jam Jars 208: Neptune’s daughter – Water.